Nowadays, MeOH exclusively is produced from the synthesis gases CO2/H2 or CO/H2, which in turn originate from the reforming of natural gas, residue oils of crude oil processing or from the pressure gasification of coal. The raw MeOH produced can directly be processed to DME or be processed by distillation to obtain pure MeOH and subsequently be catalytically converted to DME and water. In both cases, the DME product obtained is separated from unconverted MeOH and water by distillation. In general, the raw MeOH is subjected to a two- or three-stage distillation, in which first the low boilers and the dissolved gases, in particular CO2, are removed and then MeOH and water are separated, and in an adiabatic fixed-bed reactor the purified MeOH is converted to DME up to the reaction equilibrium. Since the reactor product comprises a mixture of DME, water, unconverted MeOH and minor amounts of uncondensable light gases, the reactor product is treated in a two-stage distillation, wherein in the head of the first distillation stage DME is separated from unconverted MeOH and reaction water and in the head of the second distillation stage the MeOH contained in the bottom product of the first stage is separated from the reaction water and the MeOH obtained flows back into the reactor. The uncondensable light gases discharged with the DME at the top of the first distillation stage are saturated with DME which in a gas washing stage is separated from the uncondensable gases by using MeOH as washing agent, before the same leave the system as low boilers at the top of the gas washing stage. Accordingly, both in the distillation of DME and in the distillation of MeOH mixtures chiefly comprising MeOH, water and DME are separated. Since the product specifications for DME on the one hand and MeOH on the other hand must satisfy various requirements, the distillations of DME and MeOH are carried out separately. The above-described measures are applied in particular for the production of high-purity DME, which is widely used as propellant gas, e.g. in hair spray and paint spray. Technical DME is an alternative to liquefied gases with excellent burning properties. Due to a cetane number of 55 to 60, DME can be used in diesel engines as a substitute for diesel fuel.
Since according to the Biofuels Directive 2003/30/EG of the European Parliament and the “Council for Promoting the Use of Biofuels or other Renewable Fuels in the Transport Sector” DME is regarded as biofuel, the same may contain impurities which are not allowed for high-purity DME. Therefore, the distillation of the MeOH can be omitted and the raw MeOH can directly be charged to the DME reactor. In the process described in U.S. Pat. No. 5,750,799 A, for example, untreated raw MeOH is directly introduced into a DME reactor, so that the return flow containing the MeOH is loaded with considerable amounts of water. This circulating water must be condensed in addition to the unconverted MeOH and must subsequently be evaporated before the DME reactor, whereby the energy efficiency of the process is impaired considerably. In addition, the circulating stream is increased due to the reduced MeOH conversion in the DME reactor and accordingly the apparatuses and technical components of the plant for producing DME must be designed larger. Raw MeOH contains carbonic acid H2CO3 and small amounts of organic acids which must be neutralized, in order to avoid corrosion phenomena on apparatuses and technical components made of steel, which are used for producing DME. Usually soda is used for neutralizing the raw MeOH.
U.S. Pat. No. 6,740,783 B1 describes a process for producing DME from raw MeOH by using a DME reactor with a fixed bed of zeolite catalyst which initially is deactivated by doping with metals, in order to increase the DME selectivity of the catalyst. On the long run, however, a constant supply of metals leads to a continuous deactivation and hence to a reduction of the useful life of the catalyst. The subject-matter of EP 1396483 B1 is a process for producing DME, in which raw MeOH is dehydrated in the vapor phase in the presence of an activated Al2O3 catalyst doped with Na. A limited doping with Na is important, so as not to impair the conversion of the catalyst. This means that the raw MeOH must be largely free from metal and NH4 ions. In the provided raw MeOH and its evaporation the entrainment of neutralizing agent must therefore be carefully avoided.
In the documents CN 100366597 C, CN 1830934 A and JP 2004161673 A apparatuses are described, in which raw MeOH and reflux MeOH are supplied to a common separating means. The reflux MeOH contains the entire reaction water originating from the DME reactor, so that the reflux MeOH cannot be charged to the top of the distillation column. Therefore, the use of an overhead condenser is provided, in order to lower the water content of the raw MeOH supplied to the DME reactor. This requires a considerable condenser capacity which involves a correspondingly greater performance of the reboiler, whereby the energy efficiency and the economy are reduced. JP 2004161672 A deals with an evaporator for raw MeOH, which allows a partial evaporation of the raw MeOH, wherein the non-evaporated mixture of MeOH and water together with the unconverted MeOH and the reaction water from the DME reactor is separated into process water and reflux MeOH in a separate distillation column operating at low pressure. The subcooled water-poor liquid reflux MeOH is contacted with the evaporated raw MeOH, so that the water concentration in the raw MeOH supplied to the DME reactor is lowered. Hence, it is provided to also pass non-evaporated MeOH to the reflux MeOH column along with the non-evaporated water of the raw MeOH. This measure requires the evaporation and condensation of the MeOH in the reflux MeOH column and after the return of the largely water-free MeOH to the raw MeOH evaporator the renewed evaporation of the same MeOH. Thus, a MeOH circuit without contact with the DME reactor is obtained, which consumes unnecessary energy and reduces the efficiency of the circuit.
EP 455004 A1 or U.S. Pat. No. 5,750,799 A describe washing out in a column the DME discharged with the uncondensable gases at the head of the DME column and recirculating the same to the MeOH column. This measure leads to a DME content in the raw MeOH charged to the DME reactor, by which the conversion of MeOH to DME is reduced. This is relevant in particular when raw MeOH is used and the light gases contained in the raw MeOH are not separated before the DME reactor, but only in the DME column. Proportional to the amount of the non-condensed light gases, the amount of the DME discharged with the gases and hence the amount of DME in the inflow to the DME reactor increases.
When the raw MeOH contains only little dissolved gases, the arrangement of a scrubber can be omitted. Instead, the DME discharged with the uncondensable gases can largely be recovered in an end cooler operated with cold water or a refrigerant.